1. Field of the Invention
The present invention relates to a printing apparatus capable of printing bar codes and, more particularly, to a printing apparatus for issuing a package tag including bar codes.
2. Description of the Related Art
In airports, air cargoes are managed by using package tags so as not be lost during transportation. Package tags are attached to all the air cargoes and respectively indicate their destinations and transportation means (airlines and flight numbers) as cargo data. As shown in FIG. 1, cargo data is printed on a package tag 1 as character data 2 and bar codes 3. This printing operation is performed by a package tag printer including a thermal head having a row of heating elements.
A conventional package tag printer is operated in the following manner. Printing data for one package tag is converted into bit image data and is stored in an image buffer. The bit image data is read out from the image buffer in units of lines so as to be supplied to a thermal head. Every time continuous paper is fed in a paper feed direction M by a predetermined amount, the thermal head is driven in accordance with one-line bit image data. When all the bit image data is printed on the paper, the printed portion of the paper is cut from the rest of the paper by a cutter and is issued as the package tag 1.
In this package tag 1, the angle at which each bar code 3 is printed is set such that white and black bars are parallel to the paper feed direction M, as shown in FIG. 1. In this case, each heating element of the thermal head is assigned to either a white bar or a black bar of each bar code 3. This assignment is not changed during a printing operation of the bar code 3. The heating elements corresponding to black bars are almost continuously set in an ON state while the bar code 3 is printed, and fluctuations of temperature are suppressed. Therefore, even if the thermal head is driven at a short driving interval, excellent contract can be ensured between the white and black bars. If the bar code 3 is set such that each bar code element is perpendicular to the paper feed direction M, since the respective heating elements are intermittently turned on depending on a combination of white and black bars, the temperature of each heating element may be deviated from an optimal temperature at which excellent contrast can be obtained. This leads to a deterioration in printing quality of the bar code 3.
When the package tag 1 issued by such package printer is actually attached to a corresponding air cargo and is transported to a cargo terminal of an air port, an operator at the cargo terminal urges a bar code reader against a bar code of the package tag 1 so as to cause the bar code reader to read the cargo data. This cargo data is supplied from the bar code reader to a computer system and is registered therein. In a bar code read operation, the operator must urge the bar code reader against the bar code in the proper read direction. For this reason, it is difficult to handle cargoes in large quantities within a short period of time.